The furnaces of mobile and modular homes are conventionally supplied with inlet air through an annular duct surrounding the furnace flue, as shown in U.S. Pat. No. 3,691,930. Failure to make or maintain a gas-tight connection between the flue and the furnace may not be discoverable by inspection, once the surrounding air supply duct is secured to the furnace.
Telescoping flue assemblies are used to accommodate variation of distance between furnaces and roofs. These may permit the inner flue to be pulled down simultaneously with the attached outer air supply duct. In U.S. Pat. No. 4,522,191 seating and securing the outer duct provides some assurance that the flue connection has been made. However, with such assemblies, the outer duct cannot be raised independently of the inner flue to verify the flue connections. Furthermore, the type of interconnection shown in that patent provides limited capability for dealing with the problem presented if the furnace flue and duct connections are not perfectly concentric.